Airglow, or "why is the sky green?"

Lately the clear night sky has not been as dark as I think it should be.  The reason is a phenomenon called "airglow".  Airglow is caused by molecular recombinations high in the atmosphere, such as free N and O atoms combining to form NO.  The emitted light can vary from green to red or purple depending on the molecular energy levels involved.  Airglow is a natural phenomenon and not related to skyglow caused by light pollution.

There is a widely available device for measuring sky brightness called a "Sky Quality Meter".  It is manufactured by a company named Unihedron.  The version that I have is called the SQM-L, which measures sky brightness in a narrow angular region.  Typically a very dark night at my location here in Creede will provide SQM-L values (in units of mag/arcsec²) that range between 21.6-22.0.  Over the last week or so I have been measuring values in the range of 21.0-21.2. Smaller numbers correspond to a brighter sky. At my home in Santa Fe, SQM-L readings of 19.5 are typical.  Each change of one unit corresponds to an increase or decrease in brightness of a factor of about 2.5.  This means that my sky in Santa Fe is normally about 2.5*2.5 = 6.25 times brighter than here in rural Colorado.  This difference is caused by light pollution, not airglow.

Here are some images from last night that illustrate the effect of airglow.  All images were obtained with a Sony A7iii + Rokinon 24mm f/1.4 lens + Hoya Softon-A diffusion filter. One image in each comparison is the daylight-balanced color and the other is the color-corrected version intended to mitigate the green glow.

The Pleiades and Jupiter rising above the local mountain ridgeline.

 Airglow in not uniform across the sky, so even in the color-corrected versions it is still evident at lower elevations where the line of sight through the atmosphere is longer.  The ironic aspect of airglow, which makes the sky brighter, is that detecting it requires a very dark sky to begin with, otherwise it just blends in with light pollution. 

Here is a picture from May 2022 that shows strong airglow blending with the Milky Way.  The sky brightness measurement directly overhead on this evening was 21.7.

Sony A7 + Samyang 35mm f/1.8 + softon filter. ISO 1600, 30 s.

Here is a picture of airglow visible from the International Space Station:

Credit: NASA